Identifying the Chemical Origin of Oxygen Redox Activity in Li-Rich Anti-Fluorite Lithium Iron Oxide by Experimental and Theoretical X-ray Absorption Spectroscopy

Destabilization of Oxidized Lattice Oxygen in Layered Oxide Cathode

Integrating anion-redox capacity with orthodox cation-redox capacity is deemed as a promising solution for high-energy-density battery cathodes surmounting the present technical bottlenecks. However, the evolution of oxidized oxygen species during the electrochemical or chemical process easily jeopardizes the reversibility of oxygen redox and remains poorly understood. Herein, we showcase the gradual conversion of the π-interacting oxygen (localized hole states on O) to the σ-interacting oxygen upon resting at a high voltage for P3-type Na0.6Li0.2Mn0.8O2 with nominally stable ribbon-like superstructure, accompanied by the O-O dimerization and the local structural reorganization. We further pinpoint an abnormal Li+ migration process from the alkali-metal layer to the transition-metal layer for desodiated P3-Na0.6Li0.2Mn0.8O2, thereby leading to a partial reconstruction of the ribbon superstructure. The high-voltage plateau of oxygen-redox cathodes is concluded to be exclusively controlled by the oxygen stabilization mechanism rather than the superstructure ordering. In addition, there exists a kinetic competition between π and σ interaction during the uninterrupted electrochemical process.

Synergetic Anion-Cation Redox Ensures a Highly Stable Layered Cathode for Sodium-Ion Batteries

Sodium-ion batteries are commonly regarded as a promising candidate in large-scale energy storage. Layered iron/manganese oxide cathodes receive extensive attentions due to the element abundance and large theoretical capacity. However, these materials usually undergo obvious degradation of electrochemical performance due to the tendency of Mn dissolution and Fe migration during continuous sodium release and uptake. Herein, a strategy of anion-cation synergetic redox is proposed to suppress the structural deterioration originated from overusing the electrochemical activity of transition-metal ions, and decreased lattice strain as well as superior electrochemical performance are realized simultaneously. Results show that the Na_0.8 Li_0.2 Fe_0.2 Mn_0.6 O₂ (NLFM) electrode is highly resistant to the erosion of moisture that is distinct from the traditional Mn/Fe-based electrodes. Moreover, the NLFM electrode demonstrates solid solution behavior without phase transition during cycles. The ultra-small volume change of 0.85% is ascribed to the negligible manganese dissolution and invisible transition-metal migration. The high-stable layered structure assures superior reversible capacity of ≈165 mA h g^-1 , excellent rate capability, and splendid capacity retention of over 98.3% with 100 cycles. The findings deepen the understanding of the synergy between anion and cation redox and provide new insights to design the high-stable layered cathode for sodium-ion batteries.

Engineering O-O Species in Boron Nitrous Nanotubes Increases Olefins for Propane Oxidative Dehydrogenation

Boron nitride (BN) has been widely studied as an efficient catalyst for oxidative propane dehydrogenation (OPDH). Oxygen-containing boron species (e.g., BO·, B(OH)_xO_3-x) are generally considered as the active centers in BN for OPDH. Here, we show an effective progressive substitution strategy toward the development of boron-oxygen-nitrogen nanotubes (BONNTs) enriched with O-O species as a highly active, selective, and stable catalyst for OPDH. At 525 °C, an olefin yield of 48.6% is achieved over BONNTs with a propane conversion of 64.4%, 2.8 times that of boron nitrogen nanotubes (BNNTs). Even after reaction for 150 h (475 °C), BONNTs exhibit good olefin yield. Both the B(OH)_xO_3-x and O-O species that coexist in the BONNT catalyst are demonstrated as active centers, which differs from the B(OH)_xO_3-x one in BNNTs. Based on catalytic results, propane and oxygen alternate treatment experiments, and theoretical calculations, the O-O center is more favorable for producing both propylene (C₃₌) and ethylene (C₂₌), which experiences a dehydration pathway and two possible reaction paths with a lower energy barrier to yield olefins, while B(OH)_xO_3-x is mainly responsible for producing few C₃₌.

Cation-Disorder-Assisted Reversible Topotactic Phase Transition between Antifluorite and Rocksalt Toward High-Capacity Lithium-Ion Batteries

Multielectron reaction electrode materials using partial oxygen redox can be potentially used as cathodes in lithium-ion batteries, as they offer numerous advantages, including high reversible capacity and energy density and low cost. Here, a reversible three-electron reaction is demonstrated utilizing topotactic phase transition between antifluorite and rocksalt in a cation-disordered antifluorite-type cubic Li₆CoO₄ cathode. This cubic phase is synthesized by a simple mechanochemical treatment of conventionally prepared tetragonal Li₆CoO₄. It displays a reversible capacity of 487 mAh g^-1, a high value because of a reversible three-electron reaction using Co²⁺/Co³⁺, Co³⁺/Co⁴⁺, and O^2-/O₂^2- redox, occurring without O₂ gas evolution. The mechanochemical treatment is assumed to reduce its lattice distortion by cation-disordering and facilitate a reversible topotactic phase transition between antifluorite and rocksalt structures via a dynamic cation pushing mechanism.